Hydrotalcite is a known natural mineral which is produced in relatively small amounts in limited areas. Hydrotalcite is known as an antacid which has a neutralizing effect on gastric juices. It is also known to produce synthetic hydrotalcites by the reaction of a carbonate source, a magnesium source, and an aluminum source. U.S. Pat. Nos. 3,539,306 and 3,650,704 to Kumura et al discloses synthetic hydrotalcites and methods for manufacture of the synthetic hydrotalcites. The synthetic hydrotalcite is described as having the following composition: EQU Al.sub.2 O.sub.3.6MgO.CO.sub.2.12H.sub.2 O.
According to these patents, the hydrotalcite is produced by reaction of an aluminum component, which can be a water soluble aluminate, a magnesium component which can be magnesium oxide or magnesium hydroxide, and a carbonate component such as an alkali or alkaline earth carbonate in an aqueous medium at a temperature of 0.degree.-150.degree. C. at a pH of at least 8. These patents also discuss prior art procedures for preparing synthetic hydrotalcites which include the use of dry ice or ammonium carbonate as the carbonate source and using high temperatures and or elevated pressures.
Hydrotalcites which have needle-like crystalline structures are described in U.S. Pat. No. 4,351,814 to Miyata, these hydrotalcites being prepared by reaction of a basic magnesium compound in the form of needle-like crystals with an aluminum cation under conditions which do not cause a loss of the needle-like crystalline shape of the basic magnesium compound. The reaction is carried out under a reaction temperature of about 90.degree.-150.degree. C. These fibrous hydrotalcites are said to be useful as fire retardants for thermoplastic and thermosetting resins.
U.S. Pat. No. 4,629,626 of Miyata et al discloses a hydrothermally treated product useful for treating an iron deficiency, which compound has a hydrotalcite-like crystal structure and includes iron, magnesium, aluminum and water within the structure. This product is prepared by the reaction of an iron source such as ferrous chloride with aluminum chloride and sodium hydroxides.
U.S. Pat. No. 4,347,353 to Miyata et al discloses the use of a hydrotalcite to inhibit the corrosion-causing tendency and coloration of an olefin polymer which contains a halogen component.
U.S. Pat. No. 4,611,024 to Wolfe discloses a propylene polymer composition which contains a clarifier and a small amount of hydrotalcite to enhance the effect of the clarifier.
U.S. Pat. No. 4,085,088 to Miyata et al discloses a fire retarding thermoplastic composition which comprises a thermoplastic resin and a fire retarding amount of a hydrotalcite, the hydrotalcite being coated or uncoated with an anionic surface active agent. According to these inventors, the hydrotalcite must have a BET specific surface area of not more than 30 m.sup.2 /g and a specific crystallite size in order to provide the fire retardant characteristics. U.S. Pat. No. 4,154,718, also to Miyata et al, discloses fire retardant thermoplastic compositions which comprise a thermoplastic synthetic resin, a magnesium-containing compound which can be a hydrotalcite, and a fire retardant assistant which can be an alkali metal chloride. U.S. Pat. No. 4,324,876 to Matsubura et al discloses the use of hydrotalcites as catalysts in the production of olefins.
There remains a need in the art, however, for improved hydrotalcites, novel methods for preparation of novel synthetic hydrotalcites and improvements in their characteristics as fire retardants.